In the 1980s, the first rare-earth-based catalyst systems saw the light of day and interest in them has been growing, on the one hand due to the non-toxicity of the metal and on the other hand due to the fact that non-aromatic solvents, such as hexane, cyclohexane, cuts of isomers of hexane or methylcyclohexane can be used as the polymerization solvent. More particularly, neodymium catalysis is currently experiencing a real expansion.
Rare-earth catalysts are multicomponent catalysts and are generally constituted of at least:
a conjugated diene monomer,
a salt of one or more rare-earth metals of an organic phosphoric acid or of an organic carboxylic acid,
an alkylating agent constituted of an alkylaluminium corresponding to the formula AlR3 or HAlR2, and
a halogen donor constituted of an alkylaluminium halide.
This type of catalyst is described, in particular, in International Patent Applications WO 02/38635, WO 02/38636 and WO 03/097708 in the name of the applicants.
Among the rare-earth salts, the rare-earth salts known as phosphates and of general formula Ln[OP(═O)x(OR)3-x]3 (Ln being a metal from the lanthanide family, scandium or yttrium, x being equal to 1 or 2, R being identical or different, linear or branched, aliphatic or alicyclic alkyl radicals comprising 1 to 20 carbon atoms) are significant compounds for the preparation of highly active catalyst systems for the cis-1,4 stereospecific polymerization of conjugated dienes, more particularly butadiene and isoprene. These systems have the advantage of resulting in elastomers that have a narrow molecular weight distribution.
Due to their nature, these salts are not very soluble and have a high tendency to associate in a hydrocarbon solvent medium and generally result in the formation of gel, as described by Suglobov in document “DEHP Complexes of Lanthanides (III) and Actinides (III)”, Journal of Alloys and Compounds, 213/214, 1994, p. 523-527). This leads to practical difficulties, whether during the synthesis thereof or the use thereof as a polymerization catalyst precursor.
Several methods of synthesis are described in the literature. Most consist of the reaction of a rare-earth salt of rare-earth halide or nitrate type with an organophosphate salt (Korovin et al., Russian Journal of Inorganic Chemistry, 22(5), 1977; Chen Tien et al. “Preparation and characterization of bis(2-ethylhexyl)phosphate(P204) rare earth”, K'o Hsueh T'ung Pao 1981, 26(13), 794-6; JP 60023406 in the name of Asahi, page 4).
Patent document EP 1 509 557 B1 in the name of the applicants describes the synthesis of a powder of neodymium tris(organophosphate) [(RO)2P(═O)O]3Nd by reaction of:                a) an aqueous solution of neodymium chloride NdCl3(H2O)6, prepared by reaction of HCl with Nd2O3, and        b) a water/acetone solution of sodium di(2-ethylhexyl)phosphate.        
This synthesis process, in several steps, allows a complete reaction and results in the formation of the neodymium tris[di(2-ethylhexyl)phosphate] salt in the form of a solid. After washing this solid with water a certain number of times, the qualitative test for the determination of chloride ions is almost negative, which indicates that an almost pure product is obtained. Once dry, the product is in the form of a pink-mauve powder.
Patent document U.S. Pat. No. 6,767,927 B1 describes a process for the synthesis, in three steps, of a stable solution of rare-earth tris(organophosphate) in a hydrocarbon solvent:                1) preparation of an organophosphate salt by reaction of the corresponding acid and a base,        2) reaction of the organophosphate obtained previously with a rare-earth salt,        3) adjustment of the concentration of free acid.        
This synthesis is carried out in a two-phase water/hydrocarbon solvent medium. At the end of the reaction, the water is removed by decantation and azeotropic distillation. The product of this synthesis process is a fluid solution that can be used directly for the preparation of a polymerization catalyst. Nevertheless, the process comprises a certain number of synthesis steps in order to result in the desired solution. This process furthermore generates aqueous effluents that contain ions, for example ammonium nitrate, which from an environmental point of view requires a supplementary step for the subsequent treatment of these waste products.
The preceding synthesis processes may be defined as being indirect processes. That is to say that the synthesis of the rare-earth phosphate salt is carried out starting from a rare-earth salt, obtained by reaction of the rare-earth oxide and an acid, and from an organophosphate salt, obtained by reaction of the corresponding acid and a base. The number of steps generally produces, for this type of process, complex and not very economical synthesis pathways. A drawback of these processes is that they generate aqueous waste products, which makes it necessary to introduce a supplementary step for treating these waste products.
Rare-earth tris(carboxylates), such as for example rare-earth versatates, are compounds that themselves can also be used as a precursor in the synthesis of catalysts for the cis-1,4 stereospecific polymerization. They can be synthesized according to the same pathway as before, that is to say by reaction of a carboxylate salt with a rare-earth salt (for example, reaction of sodium versatate with neodymium nitrate as described in patent document U.S. Pat. No. 6,111,082).
Alternative synthesis pathways have been developed for the synthesis of organometallic salts of this family in order to reduce the cost of the synthesis by limiting the number of steps. In these syntheses, the carboxylate salt or the rare-earth salt is replaced by the precursor thereof, that is to say the corresponding carboxylic acid or the corresponding rare-earth oxide.
Patent document U.S. Pat. No. 6,482,906 B1 describes a process for preparing neodymium tris(neodecanoate) also known as neodymium tris(versatate), in four steps:                1) preparing a sludge by dispersion of Nd2O3 in a hydrocarbon solvent and reaction with a deficiency of HCl relative to the neodymium,        2) adding neodecanoic acid in an amount of 3.25 equivalents relative to the neodymium,        3) settling of the reaction medium,        4) separating the neodymium tris(neodecanoate) from the residual reactants.        
This synthesis pathway has the advantage of not comprising the step of synthesis of the neodecanoate salt by reaction with a base, which removes one step. The drawback is that the reaction is not complete and that a step of separating the unreacted neodymium oxide from the solution of neodymium tris(neodecanoate) is necessary with a view to using the neodymium tris(neodecanoate) for other applications, especially for the preparation of a catalyst system.
U.S. Pat. No. 5,220,045 describes a process for the synthesis of neodymium neodecanoate in one step comprising the reaction, in a two-phase medium, of an aqueous solution of neodymium nitrate with an organic solution of versatic acid in the presence of a base of amine, ammonia or quaternary ammonium hydroxide type. During this synthesis, the ammonium neodecanoate salt is generated in situ in order to react with the neodymium nitrate. This synthesis pathway has the advantage of taking place in a single reactor and of being complete. It has the drawback of requiring the addition of a base to the reaction medium and of involving a certain number of washes with water and of decantation steps in order to rid the gel of its impurities, which is detrimental from an economical viewpoint: Furthermore, this process generates aqueous effluents that contain ions, for example ammonium nitrate, which, from the environmental point of view, requires a supplementary step for the subsequent treatment of these waste products.
Unlike the synthesis pathways described above, a direct synthesis consists in directly reacting the rare-earth oxide with the phosphoric or carboxylic acid, without going via the rare-earth salt and/or via the phosphate or carboxylate salt.
Patent EP 0 924 214 B1 relates to a process for fluidizing organic solutions of rare-earth compounds, such as rare-earth phosphates or carboxylates, by addition of a Lewis acid. Example 1 of this patent describes the preparation of lanthanum di(2-ethylhexyl)phosphate by reaction of lanthanum oxide La2O3 with di(2-ethylhexyl)phosphoric acid in hexane and a small amount of water. The mixture is stirred and brought to reflux until a clear yellow solution is obtained. The water is then removed by azeotropic distillation. The gel obtained is very viscous, the viscosities measured being greater than 90 000 cps, and has a brittle structure. Indeed, it is stated in this patent that holes (fracture zones) appear in the gel during the viscosity measurement. This gelatinous and “brittle” characteriztic makes it difficult to handle such a product, especially during processing steps for the preparation of a catalyst system for the cis-1,4 stereospecific polymerization of conjugated dienes, and prevents its characterization. This involves taking precautions for the handling and characterization of this product, especially during the use of this gel in an industrial process.